It's behind the distributor, on the back of the block, as trailer trash said. If I remember correctly, the GM spec for minimum oil pressure, at hot idle, in gear, is 5 - 8 psi. So 10 - 12 isn't too bad, just lower than most. The pressure transducer in the oil pressure sending unit reads by the deflection of a membrane within the unit, and given that the newest of our trucks is over a decade old, the membrane is apt to be a bit fatigued. Check the pressure against a mechanical gauge to see if the sender is off, and replace if necessary. If it IS correct, then you can either live with the lower pressure, rebuild the motor, or use a heavier oil. Personally, I would switch up to a 40 weight (Rotella T6 5W40 is a great synthetic) and call it a day. If the motor doesn't tap, click, or knock at hot idle, then it's fine.